In general, an organic electroluminescent device (hereinafter referred to as organic EL device) includes a light-emitting layer and a pair of counter electrodes interposing the light-emitting layer therebetween in its simplest structure. That is, the organic EL device uses the phenomenon that, when an electric field is applied between both the electrodes, electrons are injected from a cathode and holes are injected from an anode, and each electron and each hole recombine in the light-emitting layer to emit light.
In recent years, progress has been made in developing an organic EL device using an organic thin film. In order to enhance luminous efficacy particularly, the optimization of the kind of electrodes has been attempted for the purpose of improving the efficiency of injection of carriers from the electrodes. As a result, there has been developed a device in which a hole-transporting layer formed of an aromatic diamine and a light-emitting layer formed of an 8-hydroxyquinoline aluminum complex (Alq3) are formed between electrodes as thin films, resulting in a significant improvement in luminous efficacy, as compared to related-art devices in which a single crystal of anthracene or the like is used. Thus, the development of the above-mentioned organic EL device has been promoted in order to accomplish its practical application to a high-performance flat panel having features such as self-luminescence and rapid response.
Further, investigations have been made on using phosphorescent light rather than fluorescent light as an attempt to raise the luminous efficacy of a device. Many kinds of devices including the above-mentioned device in which a hole-transporting layer formed of an aromatic diamine and a light-emitting layer formed of Alq3 are formed emit light by using fluorescent light emission. However, by using phosphorescent light emission, that is, by using light emission from a triplet excited state, luminous efficacy is expected to be improved by from about three times to about four times, as compared to the case of using related-art devices in which fluorescent light (singlet) is used. In order to accomplish this purpose, investigations have been made on adopting a coumarin derivative or a benzophenone derivative as a light-emitting layer, but extremely low luminance has only been provided. Further, investigations have been made on using a europium complex as an attempt to use a triplet state, but highly efficient light emission has not been accomplished. In recent years, many investigations have been made mainly on an organic metal complex, such as an iridium complex, as described in Patent Literature 1, for the purpose of attaining high luminous efficacy and a long lifetime.
A device construction is also important for obtaining high luminous efficacy. For example, the following construction is given. A hole-blocking layer is laminated between the light-emitting layer and an electron-transporting layer for the purpose of trapping a hole in the light-emitting layer to increase the probability that an electron and the hole recombine in the light-emitting layer. The use of the hole-blocking layer can be expected to improve the luminous efficacy.
As described in the foregoing, both charges (a hole and an electron) need to recombine in the light-emitting layer with high probability in order that high luminous efficacy may be obtained in the organic EL device. Further, hole-blocking materials (including an electron-transporting material) themselves have been desired to be compounds each of which is electrochemically stable, and brings together high heat resistance and excellent amorphous stability, and hence further improvements thereof have been required.